The size of the linkage group depends upon the size of chromosome. The smaller chromosome will naturally have smaller linkage
group while a longer one has larger linkage group.Strength of linkage
The strength of linkage between any two linked genes depends upon the distance between them. It is calculated by the frequency
of recombination (crossing over) between them. It means the strength of linkage is inversely proportional to the distance between
the genes or the percentage of crossing over between them. There are many factors affecting strength of linkage besides distance
including many physiological as well as environmental factors such as:- Age of animal : With increasing age the chances of crossing over are reduced and, therefore, strength of linkage
increases. - Temperature : The increase in temperature increases the frequency of chiasmata formation and, therefore, it decreases the
strength of linkage. - X-rays : exposure to X-rays reduces the strength of linkage.
Sex Linkage
Sex linkage or sex-linked inheritance is the transmission of characters and their determining genes alongwith sex determining genes
which are borne on the sex chromosomes and, therefore, are inherited together from one generation to the next. The sex linkage was
discovered by Morgan (1910) when he studied the inheritance of red-white eye colour trait (locating genes on chromosomes).
Most sex-linked genes are located on the X-chromosome, forming X-linkage. a few genes occur on the y-chromosome,
forming Y-linkage. The y-linked traits are transmitted only through the male, for example, gene for sex determination in
humans. Some animals, however, may carry a few genes on the y chromosome that produce visible effects on the phenotype of
the organism. Such ‘y-linked” or holandric genes would be transmitted directly from father to son and never appear in females, y
linkage is clearly very rare in higher animals, particularly mammals.
X linkage, on the other hand, is very common in all mammals that have been studied; the mammalian X chromosome contains a large
number of genes with major effects on phenotype.
Two important sex-linked human diseases are haemophilia and colour blindness.
Sex limited traits : They are those specific phenotypic traits which are expressed in a particular sex. There are certain autosomal
genes which are expressed in one sex, not in the other due to hormonal differences or anatomical dissimilarities between male
and female sexes. Since the expression of such genes are limited to one sex, therefore they are known as sex-limited genes and
the traits controlled by them are called sex-limited traits. Genes for secondary sexual characters have sex-limited inheritance. e.g.,
plumage pattern in birds.
Sex-influenced inheritance : The expression of certain autosomal and sex-linked genes is dependent on the hormone constitution
of the individual. When they come in heterozygous condition they may be expressed in one sex not in other, e.g., pattern baldness in
human. The gene for baldness behaves as autosomal dominant in males and autosomal recessive in females.characteristics of sex-linked inheritance
- It shows criss-cross pattern of inheritance. father does not pass the sex-linked allele of a trait to his son. The same is
passed to the daughter, from where it reaches the grandson, i.e., diagynic inheritance. - Mother passes the alleles of a sex-linked trait to both sons and daughters.
- Majority of the sex-linked traits are recessive.
- Sex-linked traits are more apparent in males than in females. as many sex-linked traits are harmful, males suffer more from
sex-linked disorders. - females generally function as carriers of sex-linked disorders because recessive genes can express themselves in females
only in the homozygous state.
Traits governed by sex-linked recessive genes : - Produce disorders in males more often than in females. They express themselves in males even when represented by a single
allele because y-chromosome does not carry any corresponding alleles. - Seldom appear in both father and son.